As coating film-forming methods on car bodies, generally those for forming multilayer coating film by 3-coat-2-bake (3C2B) system comprising, after applying an electrocoating paint on the coating object, application of intermediate paint→curing by baking→application of water-based base coat paint→preheating (preliminary heating)→application of clear paint→curing by baking, have been widely adopted. Whereas, for energy-saving, attempts are made in recent years to omit the bake-curing step after application of intermediate paint and adopt 3-coat-1-bake (3C1B) system comprising, after applying an electrocoating paint on the coating object, application of water-based intermediate paint→preheating (preliminary heating)→application of water-based base coat paint→preheating (preliminary heating)→application of clear paint→curing by baking (e.g., see JP 2002-282773A).
However, in the above 3C1B system layer-mixing is apt to take place between the intermediate coating film and base coating film, and hence such problems arise as reduction in smoothness or distinctness of image of resulting coating film, reduction in chipping resistance or water resistance caused presumably by change in curing balance accompanying migration of curing agent at the interface of the coating films.